A nitride semiconductor device made of gallium nitride or the like representatively includes an LED. The LED market has been increased based on low-output LED used for a keypad of small home appliances or portable communication devices such as cellular phones, or a back light unit of a liquid crystal display (LCD). Recently, as the necessity on a high-output high-efficient light source used for interior lightings, exterior lightings, vehicle interior or exterior lightings and a back light unit of a large LCD is increasing, the LED is also being shifted to high-output products.
In devices using a nitride semiconductor, “heterogeneous” substrates such as a sapphire substrate, a silicon carbide (SiC) substrate and a silicon substrate are most frequently used for the growth of the nitride semiconductor layer. However, since these heterogeneous substrate materials have mismatched lattice constants and different thermal expansion coefficients in comparison to nitrides, the nitride semiconductor layer growing on the heterogeneous substrate contains a lot of crystal defects such as dislocations. Such defects become a main factor of deteriorating the LED performance.
The sapphire substrate has a greater thermal expansion coefficient than the nitride semiconductor layer, and thus, if the nitride semiconductor layer grows at high temperature and is then cooled, a compression stress is applied to the nitride semiconductor layer. The silicon substrate has a smaller thermal expansion coefficient than the nitride semiconductor layer, and thus if the nitride semiconductor layer grows at high temperature and is then cooled, a tensile stress is applied to the nitride semiconductor layer. For this reason, the substrate is bent, and in order to prevent the substrate from being bent, the substrate should have a great thickness. Using a thick substrate just reduces a superficial phenomenon but does not reduce the stress of the thin film. If the stress of the thin film may be reduced, a thin substrate may be used advantageously. In addition, in order to separate a chip after an LED is fabricated, the substrate should be ground with a margin of about 100 μm. In this configuration, if a thin substrate may be used, there would be a great profit in aspect of LED production.
If necessary, the nitride semiconductor layer growing on the heterogeneous substrate should be separated from the heterogeneous substrate. For this, a laser lift-off has been proposed as an existing technique. However, even though the laser lift-off method is used, the substrate may be bent and the semiconductor layer may be damaged due to a difference in thermal expansion coefficient between the sapphire substrate and the nitride semiconductor. In addition, a defect such as a crack may be easily generated at an epitaxial layer due to an impact caused by a laser beam, and further the epitaxial layer is fragile, thereby resulting in an unstable process. The laser lift-off method is accompanied with thermal or mechanical deformation and decomposition of the nitride semiconductor. This causes a loss in a grown thin film and is also inefficient in aspect of energy.
In addition, the most serious problem of the LED is low light emission efficiency. Generally, the light emission efficiency is determined by light generation efficiency (internal quantum efficiency), light emission efficiency (external light extraction efficiency) out of a device, and light amplification efficiency by a phosphor. For increasing the output of the LED, it is important to improve characteristics of an active layer in view of the internal quantum efficiency, and it is also important to enhance external light extraction efficiency of the actually generated light.
A patterned sapphire substrate (PSS) prepared by forming a pattern on a sapphire substrate is known in the art as increasing the internal quantum efficiency by reducing defects generated at the growth of the nitride semiconductor layer and also increasing the external light extraction efficiency by reducing an internal total reflection.
FIG. 1 is a diagram for illustrating a case where a nitride semiconductor layer grows using an existing PSS.
Referring to FIG. 1(a), in an existing PSS 10, a nitride semiconductor layer 20 starts growing on a bottom of a substrate and grows to cover an upper portion of a PSS lens 15 by means of epitaxial lateral overgrowth (ELO). Accordingly, as shown in FIG. 1(b), a final nitride semiconductor layer 25 having a low dislocation density between dislocated regions may be obtained.
If the nitride semiconductor layer is made of GaN, the GaN grows at 1100° C. or below. At this temperature, GaN generally grows just on the bottom due to a growth mode with a strong anisotropic property as shown in FIG. 1(a), and thus the dislocation density decreases in an area where ELO occurs as shown in FIG. 1(b), thereby improving the crystal quality.
However, if the nitride semiconductor layer is made of AlN, the above effects are not obtained. AlN grows at 1300° C. or above, higher than that of GaN. At this temperature, a growth mode with a strong isotropic property is applied. Accordingly, referring to FIG. 1(c), an AlN 30 actively grows not only on the bottom of the PSS 10 but also on a surface of the lens 15. Accordingly, the AlN epitaxial layer is highly likely to be merged before the bottom is entirely filled, and thus a void 40 is generated in the MN epitaxial layer 35 as shown in FIG. 1(d). The void 40 deteriorates the crystal quality.
Due to the above problems, it is not easy to apply the PSS to grow the AlN epitaxial layer, and thus the crystal quality deteriorates. Moreover, since the growth temperature of AlN is higher than that of GaN by 100° C. or more, AlN more seriously suffers from a stress caused by the thermal expansion coefficient, bending of the substrate or the like.
Therefore, there is demanded a method for separating a substrate with high reliability, or a method capable of obtaining a nitride semiconductor such as a high-quality nitride semiconductor substrate, a nitride semiconductor device and a nitride semiconductor layer regardless of the kind of material.